Multiferroic BaCoX 2 O 7 (X = P, As) Compounds with Incommensurate Structural Waves but Collinear Spin Ingredients
Résumé
A new paradigm in multiferroics is observed in BaCoX 2 O 7 (X = As, P) compounds. They consist of one dimensional (1D) antiferromagnetic chains undulated by incommensurate structural modulations with unusually large atomic displacive waves, giving a mixed 1D/2D "real" magnetic topology. The magnetic ground state is antiferromagnetic (AFM) with k = [½ 0 0], leading to a nonmodulated collinear spin lattice despite the aperiodic atomic framework, and allows developing spin-induced multiferroicity below T N. Severe arguments against the identified mechanisms for type-II multiferroics, i.e., by inverse Dzyaloshinskii-Moriya, exchange striction and spin-dependent p-d hybridizations, suggest an original scenario in which the atomic waves, the collinear magnetic structure, and magnetic dipole-dipole interactions may interact as crucial ingredients of the spin-induced ferroelectric phase. Here, the specific role of the Co 2+ spin-orbit coupling in the magnetoelectric (ME) phase diagram is demonstrated by comparison with the novel Heisenberg BaFeP 2 O 7 isomorph, similarly structurally modulated. This compound shows a noncollinear modulated AFM ordering, while no ME coupling is detected in its case. Accordingly, both BaCoX 2 O 7 and BaFeP 2 O 7 also undergo metamagnetic transitions above 5-6 T promoted by the modulated distribution of spin exchanges, but the spin-flop progressive alignment of the spins in the noncollinear spin structure (Fe 2+ case) turns into an abrupt flip-like transition in the uniaxial spin structure (Co 2+ case).
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